In a typical power distribution system, power is distributed over power transmission lines between a power generator and a load. The efficiency of such a distribution system improves when it is operated near unity power factor. The transmission of power, however, almost inevitably involves current induced magnetic fields, which leads to a lagging power factor of the distribution system. In addition, since loads vary at different times of the day, week and seasons, this lagging power factor is constantly changing.
In order to improve the efficiency of the distribution system, the power factor is often corrected by introducing a source of leading reactance to the system, such as capacitance. Consequently, capacitors are usually shunt-connected across the power transmission lines and can be either energized continuously or switched on and off during changing load cycles. Moreover, capacitors can either be grounded or ungrounded. In most cases, capacitors are automatically discharged when switched out of the system.
Switching of capacitors into a power system, however, may cause voltage transients, especially when capacitors are switched into high voltage systems, e.g., above 32 kilovolts, and particularly above 72 kilovolts. For example, in the case of a grounded capacitor, one side of the capacitor is usually connected to a substation ground plane. Switching such a capacitor into a distribution system may cause a current rush through the capacitor since it is usually at a different potential than the power line it is connected to. This causes voltage transients in the ground circuits of the substation, as well as voltage transients in the power line. Similarly, switching an ungrounded capacitor into a distribution system usually results in voltage transients in the power line, since the capacitor and power line are usually at different potentials.
These voltage transients may blow the protective fuses which are connected in series with the capacitor, decrease the lifespan of the capacitor, cause wear on the transmission line insulators, damage substation control circuitry, and cause interference with nearby electrical controls.
In cases of multiple phase transmission systems, these problems are multiplied. For example, in a three phase transmission system, three capacitors may have to be switched into the system, and, as each capacitor is switched into the system, it may cause transients. Accordingly, multiple phased distribution systems present additional complexities when switching capacitors into the system.
It is known that, in order to decrease these voltage transients, capacitors should be switched into the system at certain times. More particularly, in the case of grounded capacitors, the capacitor should be switched into the system when the voltage signal in the respective transmission line crosses zero potential. This is because the grounded capacitor is usually discharged, which places it around zero voltage potential. Thus, when the capacitor is switched into the system when the corresponding voltage signal on the transmission line is zero, the current rush and therefore the voltage transients are minimized. Similarly, in the case of multiple phase transmission systems, it is known that each grounded capacitor should be switched into its respective transmission line when the voltage in that line crosses zero potential.
In situations where ungrounded capacitors are used in multiple phase transmission systems, it is also known that, in order to decrease these voltage transients, the capacitors should be switched into its corresponding transmission line at predetermined times. Specifically, in cases of three-phase transmission systems, two capacitors should be switched in when the voltage potential in the two respective transmission lines are equal. This means the power system's differential potential is zero, which substantially corresponds with the potential on the capacitors (which is also about zero). This is followed by switching in the last capacitor when the corresponding voltage potential of its transmission line is at zero potential.
These synchronous timing requirements for switching both grounded and ungrounded capacitors in order to decrease voltage transients have led to the development of complex switching controls and mechanisms. For example, some known devices use a single control system for each phase of a multiple phase transmission system. Each control system monitors the voltage in its respective transmission line and attempts to switch in its respective capacitor at the appropriate time. This solution is expensive, bulky and complex. Moreover, its complexity leads to a decrease in reliability. Other known devices have complex mechanical linkages that, although controlled by one control system, attempt to mechanically delay the switching of some of the capacitors until the appropriate time. These mechanical devices are also expensive, bulky, complex, less reliable, and are difficult to maintain.